Prior to the present invention, various constructions and methods have been devised to improve the operations of superplastic and quick plastic forming equipment in the forming of high quality parts from sheets of aluminum alloy or other superplastic metal alloys. For example, in U.S. Pat. No. 5,974,847 issued Nov. 2, 1999 to Sanders et al for Superplastic Forming Process, assigned to the assignee of this invention and hereby incorporated by reference, a heated sheet of superplastic forming metal alloy is positioned on a preforming block mounted on a lower die platen. A forming die is then lowered into a preforming position in which the peripheral lower edges thereof surround the block thereof and contact the outer edge portions of the heated sheet. These lower edges in conjunction with the preform block act as stuffer unit to pull the material inwardly into an intermediate or preform position as the forming die reaches its closed or seated position. With the heated sheet in the preform or intermediate position in which more sheet material has been pulled into a preform, low pressure inert gas is injected into the interface between the preformed sheet and the preform block to stretch the sheet material into the desired shape as defined by the profile of the forming die. After part forming, the die can be opened so that the part can solidify and be handled and removed without damage thereto.